Duct Repair At 108 Year Old House

This week we replaced the ductwork in a 108 year old house in St. Elmo / Chattanooga TN. It is a great (or terrible?) example of what happens when you replace equipment without repairing or updating the ductwork. The previous owner of the house had installed a 90% Trane furnace and AC (good equipment) but left the old ductwork. The previous contractor evidently preferred to repair the ductwork rather than replace it but that was clearly a short sighted solution.

Newer furnace but ducts need repair
2004 90% furnace install but did not replace ductwork

Duct repair vs duct replacement

Our customer recently purchased this 108 year old house and asked us to price replacing the ductwork but keeping the existing furnace and AC. The conditions were not great for working in. The furnace was accessed through a hole in the bedroom floor and much of the ductwork was in tight crawlspace area. Additionally, there was extra ductwork from a previous draft furnace that was even older than the ductwork still in use. All of the ductwork had remnants of asbestos tape and there was extensive rust and gaps. There was no insulation on the ductwork and the round pipes were 9″, 10″ and 12″ diameter. It was tempting to repair the ductwork vs replace it. Just cover the pipes with insulation and try to tape over the gaps in the seams. Thankfully the customer appreciated doing it right vs “getting ‘er done.” As we got further into the job, it was clear replacing the ductwork, not repairing the ductwork was the only way to do the job right.

Problem Ductwork

Long term affects of gaps and improperly sealed ductwork can shorten the life of equipment and cause increased energy bills. The air return was in the floor and there was no holder for the filter. Even if it had been held in place to filter the air, the gaps in the ductwork downstream of the filter were large enough to put your hand through in places. The worst air quality and equipment life issue was that they had placed the furnace basically on the ground in a wet basement. This moisture rusted through the return plenum and bottom of the furnace. The furnace then pulled in wet and muddy air causing rust around the fan.

Bad duct repair, gaping hole in duct joint
Unsuccessful prior ductwork repair
Rusted out return plenum with mud in it
Return plenum had rusted and filled with mud in the bottom of it
Furnace rusted from sitting on wet ground.
Furnace sitting on ground rusted through in corner.

Bad Ductwork Causes Blocked Coil / Higher Energy Bills

Due to the rust around the bottom of the unit and fan we wanted to inspect the heat exchanger for rust and confirm the furnace was safe to use. We split the AC coil from the furnace and surprisingly the heat exchanger was in very good shape. However, the coil had taken the brunt of the poor duct condition and lack of filtering. The coil was about 75% clogged. The furnace was heating the house while sucking in moist air from the crawl space, pushing it through a blocked coil and distributing through rusted out, non-insulated ducts/ We are sure the occupants comfort level, indoor air quality and energy bills were all suffering.

Dirty AC coil before cleaning
Coil dirty from poorly installed or poorly repaired ducts
Contaminates removed from AC coil
Using a short bristled brush, the junk was removed from the upstream side of the coil to allow proper air flow.
AC coil after cleaning
AC coil after being cleaned by brush and foaming coil cleaner

Beyond Duct Repair, Time For New Ductwork

The only way to do the job right was to raise the unit up onto 4″ blocks and get it out of the dirt, which we did. However, this meant getting rid of the old plenum and ductwork (likely the reason why the last contractor put it on the dirt). We removed all of the old ductwork including the even older trunk which was no longer in use. Because there was asbestos tape involved, we used personal protective equipment (PPE) as shown in this before picture. (The after picture showed a very dirty and tired technician.)

HVAC PPE for removing old ductwork
Repairing or removing ductwork with asbestos tape requires PPE to be safe

New Ductwork Installation

After cleaning and washing the coil, we installed a new pan under the furnace to seal off the rusted bottom and then put the furnace up on bricks. We then reinstalled the coil and put the new plenum on top. It was only 13″ between the coil and the bottom of the floor joists. We had 7 supply lines to tie into the plenum. We chose to use a 12″ line to feed 3 of the 8″ supplies but even so, the plenum looked like Swiss cheese with the holes cut into it. After installing the plenum, we insulated it and put on the takeoffs. We ran flex for most of the runs. However, we did use 12″ hard pipe with 3 takeoffs from the round pipe for those three 8″ runs.

AC plenum for ductwork installation
Short AC plenum with four holes in it.
New ductwork installed on 2004 furnace
Finished. Tightly sealed ductwork will provide better efficiency and air quality.

Tips For A Quality Duct Repair or Installation

A duct installation where corners are cut and lazy practices are used will cost you a lot of money in energy bills as well as shortened life span. If the duct is not installed properly, it will get damaged easily and end up needing duct repair in just a few months or a couple of years. Here are some tips for the average Joe or Jane to be able to judge the quality of their duct repair or duct install.

Proper Flexible Duct Installation

Flexible ductwork is easy to install and takes much less time than hard pipe and insulation. However, if not done right, it can easily be damaged and leak, causing high energy bills. Flexible duct can easily be routed around and over items like pipes and wires but if pulled sideways directly off a takeoff or boot, it will tear. If only held on by tape, the tape can became old and brittle and the flex duct will disconnect. Because of this, we use an elbow whenever flex is going to be turned immediately off a metal piece and we use strong nylon straps (like zip ties) with a tool to tighten them to connect flex to the metal takeoffs, pipes and elbows.

Metal takeoff from boot box
Boot box and takeoff. If flex is connected to this and turned horizontal, the takeoff could cut the plastic flex liner.
Boot box with 90 connected for best air flow
Using an elbow to turn the corner pulling the flex insulation up over the elbow is the best way to ensure a good flex connection and proper air flow

Too Much Flexible Duct

Some states require that hard pipe be used predominately and flex duct be used only for the last 6 feet or so. This is because flexible duct is easily damaged and the corrugated inside does not help air flow efficiently. Excessive flex is the most common poor quality ductwork installation mistake we see. A box of flexible duct is 25′ long. Sometimes HVAC contractors will tape two pieces of flex together to use up scraps (which eventually come apart) or because the run is longer than 25′ long. Even if the supply line is short, using a foot or two of extra flex can cause the flexible duct to sag / hang down where it will get damaged more easily.

Properly installed flexible duct
Properly installed flexible duct will not hang down or be pulled so tight that it is about to slip off the pipes it is connected to.

Lasts but not least, during ductwork repair or ductwork install we like to have manual dampers in every supply line. This makes it easier to adjust the amount of air going into each room. While you can size the duct for the amount of air needed in the room, needs change over time. Trees grow shade, or they get cut causing more sun to come in the window. Adding equipment in a room adds heat which causes a need for more air conditioning etc. With a damper in each supply line, air can fairly easily be adjusted to reduce air in one room or add it by reducing the air going to other rooms. This costs about one or two dollars per supply but is well worth the extra cost.


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Scenic City Heating & Air

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